Change preferred Japanese coding system to UTF-8

* lisp/language/japan-util.el (setup-japanese-environment-internal):
Use utf-8 as preferred coding system instead of japanese-iso-8bit when
system is not MS-Windows. And while I'm at it, fix comment to fit current
implementation.  (Bug#28705)
Copyright-paperwork-exempt: yes.

lisp/language/japan-util.el

@@ -29,11 +29,11 @@
 
 ;;;###autoload
 (defun setup-japanese-environment-internal ()
-  ;; By default, we use 'japanese-iso-8bit for file names.  But, the
+  ;; By default, we use 'iso-2022-jp for default coding system.  But, the
   ;; following prefer-coding-system will override it.
   (if (memq system-type '(windows-nt ms-dos cygwin))
       (prefer-coding-system 'japanese-shift-jis)
-    (prefer-coding-system 'japanese-iso-8bit))
+    (prefer-coding-system 'utf-8))
   (use-cjk-char-width-table 'ja_JP))
 
 (defconst japanese-kana-table